1. Field of the Invention
This invention relates to the field of timing circuits, and particularly to timing circuits and methods for generating time intervals and/or oscillations related to a resistor and capacitor (RC) time constant.
2. Description of the Related Art
Numerous circuits exist to provide timing-related functions such as delay circuits and oscillators. Many such circuits use a resistor/capacitor (RC) network to generate voltages that ramp up or down at a rate determined by the resistance and capacitance values. A ramping voltage is compared to a threshold voltage, and a specified action is taken when the threshold is exceeded.
One possible timing circuit is shown in FIG. 1. Voltage references circuits 10 and 12 provide ‘low’ (VLOW) and ‘high’ (VLOW) reference voltages, respectively, which are provided to respective differential comparators 14 and 16. A capacitor C1 is connected between a node 18 and ground; node 18 is also connected to the other inputs of comparators 14 and 16. The output of comparator 14 drives an inverter 22. Two logic gates 24 and 26 form a flip-flop, which is driven by the outputs of the inverter and comparator 16 and provides the timing circuit's output (OUT). A current source 28 provides a current I1 to node 18 via a switch 30 which is operated by the output of gate 24, and a second current source 32 provides a current I2 to node 18.
In operation, current I2 causes a voltage on C1 to ramp down until it reaches VLOW. This condition is detected by comparator 14, which drives the output of inverter 22 low and OUT high. This toggles the flip-flop, causing switch 30 to close such that I1 is applied to C1. I1 is made larger than I2, causing the capacitor voltage VCAP to begin ramping up. When VCAP reaches VHIGH, the output of comparator 16 will toggle, as will the flip-flop, making OUT go low and switching off switch 30. The C1 voltage will ramp down again and the cycle will repeat, thereby providing an oscillator function.
However, for predictable operation, the timing circuit shown in FIG. 1 requires the use of differential comparators, as well as the provision of fixed currents and reference voltages which are all set in relationship to each other and ideally remain constant under varying supply voltage and temperature conditions. This can result in a circuit which is unacceptably costly and complex.